Tunable multi-band receiver by on-chip selectable filtering

ABSTRACT

A multiple frequency RF communications receiver is disclosed which permits greater integration on standard silicon chips and consumes less power than previous receivers. A new method for selecting the various frequency bands with a high amount of isolation and low power consumption is described. Compared to previous receiver implementations, there is no loss of selectivity in the receiver.

This application claims benefit of application Ser. No. 60/431,977 filedDec. 10, 2002.

REFERENCES

U.S. Pat. No. 6,064,866, May 16, 2000, Lange.

U.S. Pat. No. 4,972,509, Nov. 20, 1990, Maejima.

BACKGROUND—TECHNICAL FIELD OF INVENTION

The present invention relates to radio receivers and methods for thereception of RF (radio frequency) communications signals in multiplefrequency bands. In particular, it relates to integrated circuit basedradio receivers using programmable on-chip tuning.

BACKGROUND OF THE INVENTION AND DISCUSSION OF PRIOR ART

At the present time, the vast majority of RF communications receiversare of the superheterodyne type. This type of receiver uses one or moreIF (intermediate frequency) stages for filtering and amplifying signalsat a fixed frequency within an IF chain. This radio architecture has theadvantage that fixed filters may be used in the LO chain. In order forthe receiver to be useable over multiple bands, its typical architectureis as the dual-band receiver shown in FIG. 1. An RF signal arriving atan antenna 11 passes through a band-select RF filter for each band 13and 14, an LNA (low noise amplifier) for each band, 15 and 16, and intoan image filter for each band, 17 and 18, which produce a band-limitedRF signal. This band-limited RF signal then enters a first mixer foreach band 19 and 20, which translates the RF signal down to anintermediate frequency by mixing it with the signal produced by thefirst LOs (local oscillators) 21 and 22. The undesired mixer products inthe IF signal are rejected by an IF filter for each band, 23 and 24. Thefiltered IF signal then enters an IF amplifier stage for each band, 25and 26, after which the outputs are merged into the second mixer 27which translates it down to yet another intermediate frequency by mixingit with the signal produced by a second LO, 28. The signal is then sentto the baseband processing. Tuning into a particular channel within theband-limited RF signal is accomplished by varying the frequency of eachLO, 21 and 22.

In order to reduce size, power consumption, and cost, it would beadvantageous to integrate the electronic components of radio receiversand reduce the number of filters and mixers. The superheterodyne design,however, requires high quality, narrowband IF bandpass filters that aretypically implemented off-chip. These filtering components impose alower limit to the size, materials cost, assembly cost, and powerconsumption of receivers built using the superheterodyne design.Moreover, the necessity for mixer and local oscillator circuitsoperating at high frequencies contributes greatly to the powerconsumption and general complexity of the superheterodyne receiver. Inparticular, the high-frequency analog mixers require a large amount ofpower to maintain linear operation. Although many variations of thesuperheterodyne design exist, they all share the limitations of theparticular design just described.

The growing demand for portable communications has motivated attempts todesign radio receivers that permit the integration of more componentsonto a single chip. Recent advances in semiconductor processing ofinductors are allowing more and more of these filters to be implementedon-chip.

A second receiver design is the direct-conversion, or zero-IF, receivershown in FIG. 2. An antenna 57 couples a RF signal through a firstbandpass RF filter for each frequency band, 59 and 60, into a LNA foreach frequency band, 61 and 62. The signal then proceeds through asecond RF filter 63 and 64, yielding a band-limited RF signal, whichthen enters a mixer for each frequency band, 65 and 66, and mixes withan LO frequency produced by an LO for each frequency band, 67 and 68. Upto this point, the direct-conversion receiver design is essentially thesame as the previous receiver design.

Unlike the previous designs, however, the LO frequency is set to thecarrier frequency of the RF channel of interest. The resulting mixerproduct is a zero-frequency IF signal—a modulated signal at basebandfrequency. The mixer outputs, 67 and 68, are combined before beingcoupled into a lowpass analog filter 69 before proceeding into basebandinformation signal for use by the remainder of the communicationssystem. In either case, tuning is accomplished by varying the frequencyof LOs, 67 and 68, thereby converting different RF channels tozero-frequency IF signals.

Because the direct-conversion receiver design produces a zero-frequencyIF signal, its filter requirements are greatly simplified—no external IFfilter components are needed since the zero-IF signal is an audiofrequency signal that can be filtered by a low-quality lowpass filter.This allows the receiver to be integrated in a standard silicon processfrom mixer 65 onwards, making the direct-conversion receiver designpotentially attractive for portable applications.

The direct-conversion design, however, has several problems, some ofwhich are quite serious. As with the other designs described above, theRF and image filters required in the direct-conversion design must behigh-quality narrowband filters that must remain off-chip. Moreover,this design requires the use of high-frequency mixer and LO circuitsthat require large amounts of power. Additionally, radiated power fromLOs, 67 and 68, can couple into antenna 57, producing a DC offset at theoutput of mixers, 65 and 66. This DC offset can be much greater than thedesired zero-IF signal, making signal reception difficult. Radiatedpower from LOs 67 and 68, can also affect other nearby direct-conversionreceivers tuned to the same radio frequency. Furthermore, to receivesignals transmitted using modulation techniques (such as FM) in whichaccess to both the lower and upper sidebands is required, two mixers andtwo LOs are required to produce both an in-phase and a quadraturebaseband signal. Not only does this increase the power required by thereceiver, but also the phase between the two LO signals must beprecisely maintained at 90 degrees to prevent demodulation distortion.This can be difficult to accomplish with variations in temperature andother operational parameters.

Other prior art to address the multiple frequency band radios usediode-based switches to select the frequency band of interest in afilter bank as in U.S. Pat. Nos. 6,064,866[1] and 4,972,509[2]. However,these implementations require a significant number of inductor andcapacitor components, and the process of switching a diode elementintroduces an equivalent impedance element in the resonant tank, therebyreducing the quality (Q) factor of the resonant circuit, thus reducingthe performance of the receiver.

In summary, although the prior art includes various receiver designs formulti-band radios, each one has significant disadvantages including oneor more of the following: the necessity for several external circuitcomponents, the consumption of large amounts of power, poor signalreception, poor selectivity, distortion, and limited dynamic range.

OBJECTS AND ADVANTAGES OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea multiple frequency band radio receiver design, which has increasedintegration and decreased power consumption without the operationalproblems associated with previous receiver designs. It is a furtherobject of the invention to provide an equivalent performance to thetraditional multi-band superheterodyne receiver of FIG. 1.

SUMMARY OF THE INVENTION

The present invention achieves the above objects and advantages byproviding a new method for multiple frequency band RF communicationssignal reception and a new receiver design that incorporates thismethod. This method includes a method for tuning the receiver tomultiple frequency bands without loss of quality factor. In addition, itprovides a mechanism to combine the multiple frequency band signalsbefore the mixer, thus requiring only one signal path at the mixers andIF stages.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a dual-band superheterodyne receiverconsidered as prior art.

FIG. 2 is a block diagram of a direct-conversion receiver considered asprior art.

FIG. 3 is a block diagram of a receiver constructed with the principlesof the invention.

FIG. 4 is an example of an implementation of the buffer of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a block diagram of a dual-band RF communications receiverconstructed in accordance with the principles of the present invention.It includes an antenna 73 for coupling a RF signal into the input of abandpass RF filter, 75 and 76, for each frequency band. The output ofthe analog bandpass RF filters, 75 and 76, connects to the input of anLNA, 77 and 78, for each frequency band and whose output couples to theinput of an image filter, 79 and 80, for each frequency band. Thearchitecture differs from the tradition receiver at this point. Twobuffers, 81 and 82, have outputs that are combined together. Each LNA,77 and 78, and buffer, 81 and 82, have power down controls, PD1 and PD2,which enable one frequency band to be selected versus the otherfrequency band. The method of powering down the circuits enables highisolation that is not achievable in other switchable filters in theprior art U.S. Pat. Nos. 6,064,866[1] and 4,972,509[2]. The combinedoutput is then fed into the first mixer, 83, mixed with the output ofthe first local oscillator, 84. The output of the first mixer, 83, isthen fed into an IF filter, 85, and an IF amplifier, 86. The output ofthe IF amplifier, 86, is then fed into the second mixer, 87, where it ismixed with the second local oscillator, 88. The savings in thisarchitecture compared to the prior art of FIG. 1 is that one localoscillator, one mixer, one IF filter, and one IF amplifier can beeliminated at the cost of two buffers. This saves in die area and power,as well as board area if the IF filter is implemented as an off-chipcomponent.

FIG. 4 gives a possible implementation of the buffer stage as an emitterfollower. The emitter follower circuit consists of a current source,100, power down control transistors, 101 and 102, a diode-connectedtransistor, 103, and current mirroring transistor, 104, and a followertransistor, 105. The PD signal, when high, shuts down the currents intransistor, 104, and 105, thus causing the buffer to shut down andisolate its input, IN, from its output, OUT. The operation of thiscircuit is well known in the art. Additionally, those skilled in the artwill recognize that the choices available in type of buffer can includesource followers, as well as non-unity gain amplifiers. Those skilled inthe art will also recognize that the power-down signal in the LNA canalso be eliminated very little change in functionality. Additionally,those skilled in the art will recognize that more frequency bands can beadded to receiver, and that the addition of more frequency bands iswithin the scope of this patent. These and other modifications, whichare obvious to those skilled in the art, are intended to be includedwithin the scope of the present invention. Accordingly, the scope of theinvention should be determined not by the embodiment described, but bythe appended claims and their legal equivalents.

1. A multiple frequency band receiver for selecting a multiple frequencyband RF signal and having reduced number of components in a RF front endsystem, the receiver comprising: an amplifier for each frequency bandwith an output connected to an input of filter for each frequency band,wherein the output of said filter for each frequency band is coupled toan input of a buffer stage for said each frequency band, and an outputof each said buffer stage is coupled together; and a mechanism to powerdown each of the buffer stages in order to select a frequency band. 2.The receiver of claim 1 wherein the receiver comprises an architecturethat is any of a superheterodyne architecture, a low-intermediatefrequency, a direct conversion, or a quasi-direct conversion type. 3.The receiver of claim 1 wherein the output of each of said buffer stagesis connected to an input of a mixer.
 4. The receiver of claim 1 furthercomprising a low noise amplifier (LNA) for said each frequency band andeach non-selected frequency bands, wherein the receiver is configured topower down the non-selected frequency bands to improve isolation of thenon-selected frequency bands.
 5. The receiver of claim 1 wherein each ofthe buffer stages comprise emitter follower circuits.
 6. The receiver ofclaim 1 wherein each of the buffer stages comprise source followercircuits.
 7. The receiver of claim 1 wherein each of the buffer stagescomprise an amplifier topology including a low noise amplifier withpower down capability.
 8. The receiver of claim 1 wherein a number ofselectable frequency bands is an integer N, where N>1.
 9. The receiverof claim 1 wherein the filters are external components to an RF chip.10. The receiver of claim 1 wherein the filters are integrated resonantelements on an RF chip.
 11. The receiver of claim 1 wherein the receiveris implemented with CMOS, bipolar, BiCMOS, or SiGe technologies.
 12. Amethod of receiving multiple frequency bands by selecting a multiplefrequency band RF signal and of reducing the a number of components inan RF front end system, the method comprising: amplifying a multiplefrequency band RF signal for each frequency band; filtering saidamplified multiple frequency band RF signal for said each frequencyband; buffering said filtered multiple frequency band RF signal for saideach frequency band with buffer stages that have outputs connectedtogether; and powering down the buffer stages to select a frequencyband.
 13. The method of claim 12 wherein the method of receivingcomprises receiving with a receiver architecture type that comprises anyof a superheterodyne, a low-intermediate frequency, a direct conversionor a quasi-direct conversion type.
 14. The method of claim 12 whereinthe buffered and selected RF signal is mixed by a mixer.
 15. The methodof claim 12 wherein the multiple frequency band RF signal is furtheramplified by a low noise amplifier (LNA) for each frequency band and anon-selected frequency band is configured to be powered down to improveisolation of the non-selected frequency band.
 16. The method of claim 12wherein the buffer stages comprise emitter follower or source followercircuits.
 17. The method of claim 12 wherein the buffer stages comprisea low noise amplifier with power down capability.
 18. The method ofclaim 12 wherein the buffer stages comprise an amplifier topologyincluding a low noise amplifier with power down capability.
 19. Themethod of claim 12 wherein a number of selectable frequency bands is aninteger N, where N>1.